The present invention relates to a process for purifying crude aqueous 5 to 20 wt.-% hydrochloric acid which has for example been recovered from combustion gases originating from waste incineration.
Waste incineration and the recovery of aqueous hydrochloric acid from the combustion gases of the waste incineration has been known for a long time. In the past, the recovered aqueous hydrochloric acid has usually been neutralized or flocculated and then disposed of. However, the neutralization, for example with caustic, is expensive and the disposal of the produced salts is now frequently objected to by the environmentalists. Accordingly, much research efforts have been spent on possibilities of reusing the aqueous hydrochloric acid.
British patent specification 1 350 727 suggests cooling the combustion gases obtained from the incineration of halogenated organic materials and absorbing at least a part of the acid constituent(s) thereof in a cooling and adsorption zone consisting of an elongated conduit in which a continuous film of flowing water or aqueous halogen acid is maintained on the inner wall, separating the cooled gases from the liquid leaving the conduit and scrubbing the cooled gases in a scrubbing zone in order to remove any remaining acid constituent(s). The aqueous halogen acid solution is recovered in a recovery system in the desired concentration. However, the unpurified aqueous halogen acid solution usually contains an unduly high amount of solids and/or heavy metal salts.
German Offenlegungsschrift DE-A-33 34 677 relates to a process of recovering hydrogen chloride from chlorinated hydrocarbon residues, particularly those originating from the vinyl chloride production. The hydrochloric acid, which has been obtained from the incineration and a subsequent quenching cycle is first evaporated, then overheated and directly passed to an oxychlorination reactor. The described process may be useful for recovering hydrogen chloride from chlorinated hydrocarbon residues which are relatively clean and not mixed with a large amount of other components. However, when burning a blend of many different chlorinated hydrocarbon residues and, optionally, other nonchlorinated products which originate, for example from chemical production plants or municipal waste, the aqueous hydrochloric acid usually contains a high amount of solids and/or heavy metal salts. Such hydrochloric acid generally is not pure enough to be recycled to the oxychlorination reactor after evaporation and overheating.
Chemical Abstracts 82(24):158273u suggests feeding the combustion gas obtained from burning vinyl chloride waste plastic to an absorption column to recover crude HCl. The crude aqueous HCl is then evaporated and condensed to give refined HCl. Again, aqueous refined HCl which is pure enough for most enduse applications is not obtained by this process if the crude HCl contains a substantial amount of solids and/or heavy metal salts.
Since the recovered aqueous hydrochloric acid usually is not clean enough to be a useful raw material, several suggestions have been made to purify the hydrochloric acid.
Chemical Abstracts 82(10):63893 g relates to a process of burning organic wastes containing chlorides to convert the chlorides to HCl. The combustion gas is countercurrently contacted with a solvent in a condenser and the resulting concentrated solvent together with diluted HCl, recovered in the later process, is vacuum distilled to obtain concentrated HCl. The HCl-containing combustion gas from the distillation column is further washed with water to remove the remaining HCl from the gas and the resulting diluted HCl is recycled to the distillation column.
However, in many cases it would be desirable to further increase the purity of crude aqueous hydrochloric acid recovered from the combustion gases resulting from waste incineration or recovered from other sources.
German Offenlegungsschrift DE-A-38 21 482 relates to a process of purifying combustion gases containing hydrochloric acid and sulfur dioxide. The sulfate portion of the combustion gases is precipitated with calcium ions and the chloride portion is subjected to a pyrohydrolysis. The combustion gases are contacted with an aqueous suspension of magnesium hydroxide whereby the main portion of magnesium hydroxide is converted to magnesium chloride and magnesium sulfate. In the pyrohydrolysis magnesium chloride is converted to magnesium oxide and HCl. HCL is passed through an adiabatic adsorption column where 15 to 18 wt.-% hydrochloric acid is obtained. The hydrochloric acid can be concentrated to 30 wt.-% and purified in a concentrator. The German Offenlegungsschrift states that hydrochloric acid of very high purity is obtained. However, the described process requires a large amount of magnesium chloride. Furthermore, the way of purifying the hydrochloric acid is not indicated.
Accordingly, it remains desirable to provide an effective method of purifying crude aqueous hydrochloric acid, for example recovered from the combustion gases resulting from waste incineration or recovered from any other HCl containing source which method does not require the addition of a large amount of chemical compounds and by which method aqueous hydrochloric acid of high purity is obtained.